Reintroduction of an Endangered Butterfly, the Mottled Duskywing ( <i>Erynnis martialis</i> )

ABSTRACT Many butterfly species around the world face a high risk of extinction, making reintroductions to restored or improved habitats an important tool for enhancing their long‐term viability. Despite their potential value, many reintroductions either fail because a suboptimal life stage was used as the founder population, or their effectiveness is difficult to assess because of a lack of postrelease monitoring. Using detailed postrelease surveys from 2021 to 2024, we report on the outcome of the first ever reintroduction of Mottled Duskywing ( Erynnis martialis ), a federally and provincially endangered butterfly, to Pinery Provincial Park, southwestern Ontario, Canada. While reintroducing Mottled Duskywing, we released different life stages to different sites within the park. At one site, after only 4 years, Mottled Duskywing reached densities of &gt; 27 adults/ha through in situ reproduction, which was comparable to densities of the two largest extant Canadian populations &gt; 700 km away. We provide evidence that pupal and adult founders are more effective life stages than larvae for reintroduction of this species, though the use of larval founders requires further testing. Our results demonstrate that Mottled Duskywing can be reintroduced to previously restored habitats in a relatively short period of time, although additional monitoring will be required to confirm the long‐term sustainability of the population. Our study provides a framework for future reintroductions of this species and an example of how life‐stage experiments can be combined with postrelease monitoring to provide critical information for best practices of butterfly reintroductions.